Telephone system



Feb. 4, 1941. M. A. LOGAN 2,230,357

TELEPHONE SYSTEM Filed Dec. 30, 1959 METAL CON TUNER FIG.

METAL CONTAINER CR2 5: E 7 A $2 FIG. 3

' l3 //4 2; C use 1 mqll' C 5 :5 A L 1 Le a I 2 5 [L2 CD! 5 e x INVENTOR C02 MA. LOGAN ATTORNA'V Patented Feb. 4, 1941 UNITED STATES PATENT OFFICE Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application December 30, 1939, Serial No. 311,718

10 Claims.

This invention relates to telephone transmission circuits and particularly to means for preventing disturbances, such as cross-talk, in the operation of telephone communication circuits.

It is the object of this invention to reduce cross-talk between communication circuits of the type in which the ringing and talking capacitors of the associated substation circuits are housed in the same container and in which conditions necessitate the electrical association of a capacitor of one line with a capacitor of another line.

In what are known as combined hand telephone sets, it is the practice to house all of the subscribers station equipment, such as the transmission induction coil and associated condenser and the ringer and its associated condenser in the base of the telephone set. For the purpose of further conserving space, it has been found desirable to house the condensers of the talking and ringing circuits in a common container. Such a construction gives rise to natural internal capacities, such as the direct capacity between the two condensers and the capacities between each condenser and the casing in which the condensers are housed, which must be reckoned with in the design of certain telephone facilities, such as those furnished by key telephone sets.

The key telephone set provides facilities which render it possible for a subscriber to utilize his telephone equipment in answering or initiating calls on lines other than the line to which his equipment is normally connected. In order that the subscribers line from which the station equipment has been transferred while a call on another line is being handled, be made available for use by another subscriber having the same facilities, it has been found practicable to interpose the switching equipment, or keys, between the ringer bridge and the speech transmitting and receiving equipment of each line so that when such keys are operated, the ringer circuit is undisturbed in so far as its connection to the line is concerned and only the speech transmitting and receiving circuits are switched to the other line. a It follows, therefore, that under such conditions wherein subscribers are utilizing lines other than the ones to which their talking equipment is normally connected, the ringing capacitor of each line is electrically associated with a talking capacitor of the other line. Thus a condition conducive to crosstalk between lines results due to the capacitive coupling between capacitors housed in the same container but electrically associated with difierent lines.

In accordance with a feature of this invention,

this condition of cross-talk is minimized by conductively connecting the container, which houses the capacitors of the line circuit with a particu. lar conductor of the line with which it is associated and by so designing the capacitor nous ing that a particular relationship exists between the direct capacity between the talking and ringing capacitors, and the capacity between the ringing capacitor and the case. More specifically, the internal separators, of the condensers and their housing are so dimensioned as to make the direct capacity between the two condensers equal to the capacity between the ringing condenser and the casing, and an electrical connection is extended ,from the casing to the line conductor opposite to the one in which the talking condenser is included.

The invention will be readily understood from the following detailed description made with reference to the accompanying drawing in which:

Fig. 1 is a simplified key equipment telephone circuit involving the features of the invention;

Fig. 2 is a circuit schematic illustrating the capacity coupling condition whichprevails when the keys of Fig. 1 are actuated to connect the subsets to alternate lines, and Without the casing connection to the line conductor. This figure also shows the various capacities which must be Ireckoned with in solving the problem of crosstal Fig. 3 is a modification of Fig. 2 showing the distribution of the objectionable capacities when the containers are connected to a line conductor; and

Fig. 4 is a circuit schematic evolved from Fig. 3 and illustrates more clearly the manner in which the objectionable capacities are distributed by connecting the capacitor container to one of the line conductors.

The simplified key equipment telephone system illustrated in the drawing comprises but two stations A and B. Each station includes the usual induction coil, transmitter, receiver, transmission condenser and the signaling equipment comprising a ringer and ringing condenser. The numerals Ill and II identify the transmission condenser and ringing condenser, respectively, at station A while the numerals 20 and 2| identify thecorresponding elements at station B. The condensers it and H at station A are housed in a single container 30 whereas the single container 3! houses the condensers 2B and 2| of station B.

The substation equipment at station A, BX: cluding the ringing condenser H and its associated ringer, is normally connected to the line conductors of line LA by way of the normal contacts of key KA. The substation equipment at station B, excluding the ringing condenser 2I and its associated ringer, is normally connected to the line conductors of line LB by way of the normal contacts of key KB. The alternate contacts of key KA are electrically connected to the line conductors of line LB whereas the alternate contacts of key KB are electrically connected to the line conductors of line LA. The ringing circuit of each line is permanently connected across the conductors of its respective line on the central oflice side of the respective switching keys KA andKB.

A call originating at the central oflice, or at some other outside point, on either of the lines LA or LB will result in the actuation of the ringer associated with the line on which the call is incoming, in accordance with well-known practice and the party whose substation equipment is normally associated with the line whose ringer is actuated will respond to the signal in the wellknown manner.

In the event that the party for whose station the call is intended is not available, the other party may pick up the call. For descriptive purposes, it will be assumed that a call incoming on line LA and intended for th party at station A who is not available to answer the call, is picked up by the party at station B. To effect such a pick-up, the party at station B actuates the key KB which transfers the speech receiving and transmitting equipment from the line LB to the line LA, and proceeds to answer the call in the usual manner. It will be noted that the-actuation \of key KB does not remove the station ringer and its associated condenser 2| from line LB.

Should the party at station A return and desire to use his telephone while the party at station B is utilizing the line LA, or should the ringer associated with line LB be actuated to indicate a call incoming on line LB while the party at sta tion B is conversing on line LA, the party at station A may actuate the key KA to transfer his speech receiving and transmitting equipment to the line LB and thus utilize the line LB for communication purposes. The actuation of key KA does not remove the ringer and its associated condenser II from the line LA.

Under the assumed conditions, the party at station A is conversing on the line LB while the party at station B is conversing on line LA. It will also be noted that condenser -I0, which is housed with condenser I I in container '30 is ass1o ciated with line LB, Whereas the condenser III is still connected to the line LA. In like manner, the condenser 20 is associated with line LA while the condenser II, which with condenser 20 is housed in the container '3I is connected to the line LB. Unless provision was otherwise made, the direct capacities between condensers I0 and I I, which condensers are now associated with different lines, and the capacities between each condenser and the container 30, as Well as the direct capacity between condensers 20 and 2|, which condensers are'now connected to different lines and the capacities between each of these condensers and the container 3| would produce an electrostatic coupling of the lines LA and L With the lines LA and LB electrostatically coupled in this manner a condition of cross-talk would prevail which would manifest itself as disturbance to the conversing parties. In efiect, the conversation being carried on over line LA would be e eard y the p r ies conversing line B and the conversation carried on over line LB would b audible to the parties conversing on line LA.

To eliminate this condition of cross-talk, each condenser housing is conductively connected to one of the line conductors of its respective line. These connections are indicated on the drawing by the numerals 13 and 23. The connection to the condenser housing may be made by soldering or in any other suitable manner.

The following discussion is made with reference to Figs. 2, 3 and 4 and particularly to Fig. 4 and constitutes an exemplification of the manner in which cross-talk is overcome by connecting the containers 3!] and L3I (Fig. 1) to the points I4 and 24 respectively of each conductor of the lines LA and LB and by making the direct capacities, CB1 and CB2, between the talking and ringing condensers I0 and I I and 2|] and 2| respectively, equal to th capacity CR1 and CR2 between the ringing condensers II and 2I and their respective containers 3!] and 3 I In this discussion:

CB1=the capacity between ringing condenser II and the container 30 CB2=the capacity between ringing condenser 21 and the container 3| Cr1=the capacity between talking condenser I0 and the container 30 CT2=the capacity between talking condenser 20 and the container 3| Cn1=the direct capacity between ringing condenser I0 and talking condenser I I CB2=the direct capacity between ringing condenser 20 and talking condenser 2II L=the line impedance of the lines LA and LB S1=the subset impedance of the subset at station A S2=the subset impedance of the subset at station E=talking voltage I1=current in subset impedance S1 In the design of communication circuits of this type it is essential for maximum efficiency in the operation of the substation circuits that the subset impedance S1 approximately equal the line impedance L and also the subset impedance 82 approximately equal the line impedanc L, it being understood that the line impedance of the two lines LA and LB are approximately the same.

With particular reference to Figs. 3 and 4 it will be noted that by connecting the container 30 to the point I 4 of conductor L1 of line LA the total capacity Cm fnom the talking condenser II) to the case 30 is connected directly across the tip and ring conductors of the line LA so that it no longer effects cross-talk. The same condition prevails with respect to GT2 and the line LB. The capacities CR1 and CR2, from the ringing condensers to the containers 30 and 3| are connected across conductor L1 of line LA and conductor L2 of line LB and across conductor L1 of line LB and conductor L2 of line LA, respectively, and the direct capacities CD1 and CD2 are connected acnoss conductor L2 of line LA and conductor L2 of line LB. With this circuit arrangement these are the only capacities which serve to produce cross-talk and are the electrostatic couplings between the lines LA and LB.

In the following discussion it will be assumed that the subscriber at station B is conversing with another subscriber over the line LA and that subscriber at station A initiates a call over line LB. The station A, under these conditions becomes the disturbed station and it need only be proved that the cross-talk current in the subset impedance S1, which represents the disturbed station A, is zero, in order to show that cross-talk is eliminated.

The source of speech currents in Fig. 4 is indicated as two independent sources with their junction connected to ground. This showing is accurate since, in considering crosstalk, we are concerned only with the alternating current component of the central ofiice battery and this battery, as shown in Fig. 1, is connected between both line conductors of line LA with one terminal grounded, so that one-half the line impedance L appears on either side of the talking battery.

In proving the cross-talk current in S1, the dis turbed set, is zero, each voltage to ground and the resultant current which flows through the disturbed set S1 will be considered independently of other voltages and currents and finally, the algebraic sum of all the currents through S1 will be computed. This method of demonstration of the invention depends upon the Superposition Theorem set forth on page 50 of the 1929 edition of Transmission Networks and Wave Filters by T. E. Shea, published by D. Van Nostrand Co. Inc.

The speech voltage E causes a current I to flow in the wiring plan as indicated by the curved arrow in Fig. 4. The value of this current may be represented by the following equation:

E L-l-S but L:Sz as above indicated, therefore Next, the voltage to ground at point 24 will be determined. This voltage may be represented by the following equation:

The equivalent voltage to ground from point 24 causes a small current to flow through CR2 to the point l5 and from this point to ground by two paths, one including the subset impedance S1 and the impedance of the line LB and the other including only the impedance Ejw CR, 11 100,,2

In a similar manner, it is determined that the current through CR1 caused by the voltage appearing between the point 25 and ground is E 1 E C 0121 j 1 As in the previous case, the part of this current which flows through S1 is one-quarter of EjwC which current flows from It to 15, that is, in the opposite direction to Iona The current through S1 due to Caz and CR1 is, therefore,

E C E I81: JZOG 122 ji gzzi) ME since O =C g The current through CD1 due to the voltage from the point 25 to ground is E 1 EjwCm 4 j or (13) In like manner the current through CD2 due to the voltage from the point 25 to ground is E 1 EjwC 4 jwcm or (14) The total current to the point l5 through CD1 and CD2 is therefore,

This current divides at point 25 and one-quarter of it flows through S1 in the same directionas ICRZ- This value of current is then By algebraically adding Equations (12) and (1'7) the total current through S1 is Since CR is equal to CD the value of the current through S1 is zero, thus showing an absence of cross-talk current in the substation circuit S1 which corresponds to the substation circuit at station A.

It is clear from the above discussion and computation that the electrostatic coupling between the two lines is caused by CR2, CR1, CD2 and Cm and the value of the cross-talk current in S1 due to this coupling is zero. Hence, cross-talk between the two lines is eliminated by making the direct capacities between the ringing and talking capacitors equal to the capacities between the ringing capacitors and their respective containers. It is also understood that the two units, each involving a ringing capacitor and a talking capacitor are identical so that CR1 equals CR2 and CD1 equals CD2- Though the above discussion has been confined to proving an absence of cross-talk current in S1, by an identical procedure the value of crosstalk current in S2 due to speech currents in line LB may also be shown to be zero.

What is claimed is:

1. A subscribers telephone set comprising in combination a speech transmitting and receiving circuit and a signal circuit, a metal container, a condenser included in said speech transmitting and receiving circuit and housed in said container, a condenser included in said signal circuit and housed in said container, said condensers beng so separated one from the other and from the container that the capacity between said second condenser and said container equals the direct capacity between the condensers, and a connection from said container to a terminal of said speech transmitting and receiving circuit whereby the internal capacity between said first condenser and said container is efiectively connected across the terminals of said speech transmitting and receiving circuit.

2. In a telephone system, a pair of telephone lines each having a speech transmitting and receiving circuit including a condenser normally connected to the line, a signaling circuit including a condenser permanently connected to the line and a metal container housing the condensers, means for transferring the speech transmitting and receiving circuit of each line to the other line whereby the condensers housed in the same container are electrically included in different lines and the said lines are thus electrostatically coupled, and means for eliminating cross-talk resulting from such coupling comprising a conductive connection between each of the containers and a line conductor of its associated line circuit.

3. In a telephone system, a first telephone line having a telephone set comprising talking and ringing equipments each containing a capacitor, a second telephone line having a telephone set comprising talking and ringing equipments each containing a capacitor, a metallic container for each group of capacitors of each line, switching means for connecting the talking equipment of said first line with the ringing equipment of said second line and the talking equipment of said second line with the ringing equipment of said first line whereby the capacitors housed in the same containers are effectively included in difierent lines, and means for reducing cross-talk between the two lines due to the capacitive coupling between the capacitors included in different lines but housed in the same container, said means comprising a conductive connection from each container to a conductor of its respective line.

4. In a telephone system, a line circuit comprising two sections interconnected by a switching device, a ringer circuit including a capacitor connected across one of said sections and the other section terminating in speech receiving and transmitting equipment including a capacitor, said two capacitors being housed in the same metallic container, a second line comprising two sections interconnected by a switching device, a ringer circuit including a capacitor connected across one section of said second line and the other section thereof terminating in speech receiving and transmitting equipment including a capacitor, the capacitors of said second line being housed in the same container, means including cross-connections between said lines for connecting the speech receiving and transmitting section of said first line with the ringer section of said second line and the speech receiving and transmitting section .of said second line with the ringer section of said first line upon the actuation of said switching devices, whereby the capacitors housed in the same containers are connect-ed in different line circuits and the different line circuits are electrostatically coupled and means for reducing cross-talk between the two lines due to the electrostatic coupling, said means comprising a conductive connection from each of the capacitor containers to a similar conductor of their respective line circuits.

5. In a telephone system, a pair of lines each having a pair of capacitors connected thereto and housed in a single metallic container, a connection involving said lines in which one of the capacitors in each of said pairs is electrically disconnected from its own line and connected to the other line whereby said lines are electrostatically coupled by the capacities existing between each of the capacitors and their respective containers and the direct capacity between the capacitors housed in the same container, and means for reducing cross-talk produced by the electrostatic coupling, said means comprising a connection from each of said containers to one of the line wires of its respective line.

6. In a telephone system, a pair of telephone lines each having a ringer circuit and a speech receiving and transmitting circuit and each of said circuits including a capacitor, a metallic container housing the capacitors of each line, means for interconnecting said lines so that the ringer circuit of each of said lines is associated with the speech receiving and transmitting circuit of the other line whereby said lines are electrostatically coupled by the capacities existing between each of the capacitors and their respective containers and the direct capacities between the capacitors housed in the same containers, and means for reducing cross-talk produced by said coupling, said means comprising a connection from each container to one of the conductors of the line with which the containi ers are associated.

7. In a telephone system, a telephone line having a speech transmitting and receiving circuit including a condenser and a signal circuit including a condenser, a metal container housing said condensers, said first condenser effecting a capacity CT with said container, said second condenser efiecting a capacity Ca with said container and said two condensers effecting a direct capacity CD therebetween, a second telephone line, switching means in said first line for transferring the speech transmitting and receiving circuit thereof to said second line whereby the condensers housed in said container are electrically included in difierent lines, and a connection from said container to a line conductor of said first line whereby the direct capacity CD is connected between similar conductors of the two lines, the capacity Ca is connected between dissimilar conductors of the two lines and the capacity CT is connected across the two conductors of said first line when said switching means is operated.

8. In a telephone system, a telephone line having a speech transmitting and receiving circuit including an induction coil and a condenser, and a signal circuit including a condenser, a metal container housing said condensers, said first condenser efiecting a capacity CT with said container, said second condenser efiecting a capacity CR with said container and said two condensers effecting a direct capacity CD therebetween and said condensers and said container being so relatively disposed that the direct capacity CD equals the capacity CR, a second telephone line, switching means in said first line for transferring the speech transmitting and receiving circuit thereof to said second line whereby the condensers housed in said container are electrically included in clifierent lines and a connection from said container to a point on a conductor of said first line intermediate said switching means and the induction coil of said first line whereby the direct capacity CD is connected between similar conductors of the two lines, the capacity Ca is connected between dissimilar conductors of said lines and the capacity CT is connected across the two conductors of said first line when said switching means is operated.

9. In a telephone system, a pair of telephone lines, each line having a speech transmitting and receiving circuit including an induction coil and a condenser and a signal circuit including a condenser, a separate metal container housing both the condensers of each of said lines, said first condenser of each line effecting a capacity CT with its respective container said second condenser of each line efiecting a capacity CR with its respective container and said two condensers of each line efiecting a direct capacity CD equal to the capacity CR, switching means for each of said lines for transferring the speech transmitting and receiving circuit thereof to the other line whereby said first condenser of said first line and said second condenser of said second line are electrically included in said second line and said second condenser oi. said first line and said first condenser of said second line are electrically included in said first line, and a connection from each of the containers to the line terminal of the induction coil of the speech transmitting and receiving circuit of its associated line whereby, when said switching means are operated, the direct capacities CD are connected between similar conductors of the two lines, the capacities Ca are connected between dissimilar conductors of the two lines and the capacities CT are each connected across the two line conductors of separate lines.

10. A subscribers telephone set comprising a speech transmitting and receiving circuit, a signal circuit, a condenser unit comprising a pair of condensers housed in a metal container and so disposed relative to each other and to the container that the capacity between one of said condensers and the container equals the direct capacity between the condensers, the said one of said condensers being included in said signal circuit and the other condenser being included in said speech transmitting and receiving circuit and a conductive connection between the container and a terminal of said speech transmitting and receiving circuit.

MASON A. LOGAN; 

